How to Dual-boot Windows 10 with Windows 7 or 8 OS

Firstly, 

Don't install windows 10 on your primary PC. But if you did, you should at least install it in a dual-boot configuration. The reboot to switch between your installed versions of windows.

Make backups of your important files before doing this thing. You will not lose your files if you follow this process.

Note: A mistake or bug could cause you to lose them.

UPDATE: Be informed if you haven't installed windows 10 on your pc before, you will have tp perform an upgrade first before you can clean install.

Resize your windows 7 or 8 partition to make space

First, you will need to make space for windows 10 on your hard drive. but for those with two hard drives in your computer and one of them is empty you can skip this part. But you will want to install windows 10 alongside windows 7 or 8 on the same hard drive.

If you are using windows 7 or 8, you can use the disk management utility to do this .Press windows key + R, type diskmgmt.msc into the run dialog, and press enter to launch it.

Locate your system partition  that's the C: partition Right click it and select "shrink volume". If you have multiple partitions on your hard drive, You can also choose  to resize a different partition to free up space.

Shrink the volume to free up enough space for your windows 10 system. Microsoft rates windows as the same and the 64bit version of windows 8 . It requires at least 20gb or more of hard drive space.

After shrinking the partition, you can continue the process.

Download windows 10 and boot installer 

Download a windows 10 iso file and either burn it into a DVD or make a bootable USB flash drive.

Leave the DVD or bootable USB in your computer and let it boot till it automatically boot into the windows 10 installer. If it doesn't boot you may change the boot order in your bios.

Install windows 10 Alongside windows 7 or 8 

 Install windows 10 process normally select your language and then click "Install now"

After agreeing to the license agreement, click the "custom install windows only [advanced]" installation option. Upgrading will upgrade your existing window 7 or 8 system to the windows 10. Technical preview custom. let you install windows 10 alongside an existing copy of windows

You will be taken to "where do you want to install window" screen, which handles partitioning. Then you will see "unallocated space" option here, assuming you resized your existing windows partition to free up space earlier. Select it and click new to create a new partition in the empty space.

A size box will pop up asking how big you want the partition to be.By default it will take up all the available unallocated space, so just click apply to make new partition using all that space.

The windows installer will create a new partition and select it for you . Click next to stall windows 10 on the new partition.

Windows will finish installing normally without asking you any question.